“We’re aiming for 5% higher yields with more and bigger fruits”

The roofs of many new greenhouses now contain glass with an anti-reflection (AR) coating, which improves light incidence. For a long time there was no such solution for existing greenhouses. But that has changed. Van Adrichem Kwekerijen is one of the first cohort of companies to gain experience with this new coating. The result: yields are up by 5%.

When sunlight falls on the greenhouse roof, three things happen. Much of it passes through the glass; this is transmission. Some reflects off the greenhouse roof and is therefore lost to the crop. This is reflection. The third thing that happens is absorption, which causes the material to warm up slightly. Together, transmission, reflection and absorption always make up 100% of the solar radiation that falls on the greenhouse roof. There is not much that can be done about absorption; that’s simply a property of the material. So when you’re looking to increase transmission, reducing reflection is a good place to start.

The discovery that applying an anti-reflection coating impacts positively on the crop was made in research done by Wageningen University & Research (WUR) back in 2006. Since then, growers installing new greenhouses have almost always had the glass factory-coated with an AR coating.

Practical experiences

Twelve years on and there is now a solution available for existing greenhouses as well. Measurements by the WUR LightLab have shown that the effect of this coating is comparable to that of AR glass. So an anti-reflection coating is an option for everyone.

The results of the first practical experiences were awaited with great anticipation, and they are now in. In March 2017, Van Adrichem Kwekerijen had a Mardenkro AntiReflect coating applied to an unlit section of its greenhouse in Steenbergen near the southern Dutch border, where they grow the large cluster tomato Merlice. “We wanted to see for ourselves what the effect would be, so we went about the process very rigorously,” commercial manager Loek van Adrichem says. “We had one 1-hectare section coated and left the one next to it untreated. We also selected 20 rows for comparison in each section. They were in the middle to rule out any impact from the side walls.”

Nothing at all happened to begin with. No matter how closely van Adrichem monitored the crop, he really didn’t see any differences. Not in leaf colour, leaf size, vegetative or generative growth, or fruit development. “Our cultivation operations were exactly the same, and in both sections, which can be controlled independently of one another, we did exactly the same with the climate, screens, ventilation and so on, because there was no reason to do things differently. Also, we had a bit of tomato russet mite here and there but we saw no differences between the rows in the weekly counts.”

Get coating in early spring

But gradually something began to catch the eye in the row records: yields in the coated section were rising compared with the uncoated section. Van Adrichem: “By July we were 2% up. By mid-October this had risen to 5% in the 20 rows we were measuring. This was a pleasant surprise. Because we prune all the trusses down to five fruits, we didn’t actually pick more tomatoes, but the fruits were slightly bigger. The only caveat is that the tomato russet mite infestations may have distorted the picture a little. That could narrow the difference slightly.”

In fact, it would have been better to apply the coating immediately after planting in December. But that wasn’t possible from a technical point of view as it was too cold and wet. “As a rule, the extra light would have the most impact on young plants,” he says. “So we lost three months of light gain because we applied the coating late. We mainly picked more kilograms in the summer, which is a bit of a pity. But in the autumn it also added up nicely. And that’s a good thing, as this isn’t a cheap solution. It needs to earn its keep. If you pick more kilograms in both the spring and the autumn, it’s a very attractive proposition.”

Light measurements

When the coating was applied in Steenbergen, six individual glass panes were also sprayed. The WUR LightLab measured their light transmittance. Compared with untreated samples, they allowed 3.6% more hemispherical light through.

This year the manufacturer plans to measure the effect in the greenhouse setting using specially designed measuring units. This is complicated because measuring PAR light under practical conditions is very difficult; there’s a reason why WUR carries out all its measurements under standardised conditions.

Meanwhile, more than 50 hectares of glass in the Netherlands have been treated with the new anti-reflection coating. The nurseries concerned grow tomatoes, sweet peppers, roses, chrysanthemums and lilies. In all cases, Mardenkro was present when the glass was coated and it also carries out light measurements. “We do this because the application process is a lot more precise than for other coatings,” says account manager Paul van Gils. “Only contractors certified by us can work to our strict protocol and the weather conditions have to be right. We then guarantee an increase in light incidence of at least 2.5%, but in practice we are seeing averages of more than 3%.”

Production up by 5%

In 2016, the research institute calculated what the crop could do with this extra light. A range of increases in production were arrived at: 3% in tomatoes, 3% in sweet peppers, 5% in cucumbers, 2.5% in roses, 2.5% in gerberas and 3% in chrysanthemums. Van Adrichem is therefore well above the calculated figure ¬– thanks in part to his green fingers.

His intention this year is to scale up to 200 hectares in the Netherlands, Belgium, Germany and the UK, again with strict oversight during application. Meanwhile, measurements taken by the laboratory show that after one year, the degree of light transmittance is still more or less maintained.

Van Adrichem is continuing with the same coating for now. He is waiting for the results from the current season and will then decide how to proceed. “If we pick slightly bigger fruits from the outset and achieve higher yields early in the season, then 5% is a nice figure to aim for.”

Summary

Van Adrichem Kwekerijen has been using a new anti-reflection coating that can be applied to existing greenhouses for almost a year now. The coated glass lets 3.6% more light through. The nursery has achieved a 5% increase in production with Merlice tomatoes and is continuing the trial with the same coating this season.